Considerable research has been devoted over the years to provide improved corrosion protection to metallic surfaces. One way of providing this corrosion protection is by electrodepositing a zinc coating on the surface. For decades, electroplated zinc has been used by the automotive industry to provide an economical, highly corrosion-resistant coating. However, with continued demands for higher quality and extended warranties, both the automotive manufacturers and their suppliers have had to develop new coatings. The best overall performance is being demonstrated by zinc-cobalt and zinc-nickel alloy platings. These alloys are being used as replacements for conventional zinc electroplates in automotive as well as other applications requiting extended corrosion-resistance. The term "alloy," as used in this specification and claims is defined as a mixture of two or more metallic elements which may be microscopically homogeneous or microscopically heterogeneous.
The improvement of zinc-nickel alloys has been demonstrated by superior salt spray performance when comparing zinc-nickel to zinc electrodeposits. The amount of nickel in the zinc-nickel electrodeposit that is useful for improved corrosion protection has been found to be from about 4% to about 18% nickel with an optimum level of about 10% to 12%.
Typically, acid zinc-nickel alloy plating baths have been based on inorganic zinc and nickel salts such as zinc sulfate, zinc chloride, nickel sulfate or nickel chloride, and the baths contain various additives to improve the brightness and the grain structure of the deposit and provide control of the zinc to nickel ratio.
U.S. Pat. No. 2,876,177 (Grundel et al) describes nickel electroplating baths containing internal salts of quaternary ammonium-N-alkyl sulfonic acids wherein the electroplating baths are Watts-type acid nickel electroplating baths. Acid zinc-nickel alloy plating baths generally contain an acid such as boric acid or sulfuric acid and other additives such as brightening agents, wetting agents, etc. U.S. Pat. No. 3,862,019 (Rosenberg et al) describes an aqueous acid electroplating bath which contains nickel salts and as brightening agents, the synergistic combination of N-(3-sulfopropyl) pyridinium inner salt and an acetylenic alcohol-ethylene oxide adduct.
U.S. Pat. No. 4,421,611 (Cameron) describes an aqueous acidic plating bath for the electrodeposition of nickel or a nickel-iron alloy which comprises nickel ions or a mixture of nickel ions and iron ions, certain acetylenic acid compounds and, optionally, an aromatic heterocyclic nitrogen-containing compound generally referred to as sulfo-betaines.
Aqueous alkaline zinc-nickel alloy plating baths also are known and have been described in the art. For example, U.S. Pat. No. 4,861,442 (Nishihama et al) describes aqueous alkaline baths comprising zinc and nickel ions, alkali metal hydroxide, an amino alcohol polymer, a nickel complexing agent, and an amino acid and/or a salt of an amino acid. The pH of the bath is 11 or higher.
U.S. Pat. No. 4,877,496 (Yanegawa et al) describes aqueous alkaline baths comprising zinc and nickel ions, an alkali metal hydroxide, a metal complexing agent, a primary brightener, and a booster brightener. The primary brightener is a reaction product of an amine such as ethylenediamine with epihalohydrin. The booster brightener is at least one aromatic aldehyde. Tertiary brighteners such as tellurium oxide, tellurous acid or its salts or telluric acid and its salts also can be included in the baths.
U.S. Pat. No. 4,889,602 (Oshima et al) describes aqueous plating baths having a pH of more than 11 and comprising zinc and nickel ions, and at least one compound from the group consisting of (i) aliphatic amines, (ii) polymers of aliphatic amines, or (iii) a compound selected from the group consisting of hydroxyaliphatic carboxylic acids and salts.